<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f(1)">
<input type="button" value="减" onclick="f(2)">
<input type="button" value="乘" onclick="f(3)">
<input type="button" value="除" onclick="f(4)">
<div>结果:</div>
<script>
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let d = document.querySelector("div");

    function f(x) {
        if (isNaN(i1.value)||isNaN(i2.value)){
            d.innerText = "输入错误!";
            return;
        }
        switch (x) {
            case 1://加
                d.innerText = "结果:"+(i1.value*1+i2.value*1);
                break;
            case 2://减
                d.innerText = "结果:"+(i1.value-i2.value);
                break;
            case 3://乘
                d.innerText = "结果:"+(i1.value*i2.value);
                break;
            case 4://除
                d.innerText = "结果:"+(i1.value/i2.value);
                break;
        }
    }

    function f1() {
        if (isNaN(i1.value)||isNaN(i2.value)){
            d.innerText = "输入错误!";
            return;
        }
        d.innerText = "结果:"+(i1.value*1+i2.value*1);
    }
    function f2() {
        if (isNaN(i1.value)||isNaN(i2.value)){
            d.innerText = "输入错误!";
            return;
        }
        d.innerText = "结果:"+(i1.value-i2.value);
    }
    function f3() {
        if (isNaN(i1.value)||isNaN(i2.value)){
            d.innerText = "输入错误!";
            return;
        }
        d.innerText = "结果:"+(i1.value*i2.value);
    }
    function f4() {
        if (isNaN(i1.value)||isNaN(i2.value)){
            d.innerText = "输入错误!";
            return;
        }
        d.innerText = "结果:"+(i1.value/i2.value);
    }

</script>
</body>
</html>